It is exactly that. Some extensions (it used to be "many extensions" before E10S was enabled on the Nightly and Aurora channels) assume that they can call back and forth between page content and browser chrome, which is easy when they're all JS objects in the same memory space, but which doesn't work nearly as well between processes.

It might be worth downloading Firefox Developer Edition (which uses a separate profile) and installing your usual suite of extensions to check they all still work.

The only extension I personally know is weird under E10S is "It's All Text", where the edit button doesn't appear on textareas until you switch to a different tab and back. Other extensions like uBlock Origin, Tree Style Tabs, Open In Browser, HTTPS Everywhere, Document Font Toggle all work fine.